1. Technical Field
This invention relates to the art of AM radio broadcasting and more particularly to an RF power amplifier system of the type employed in AM radio broadcasting together with improved driver circuitry for driving the power amplifiers.
2. Description of the Prior Art
The U.S. Patents to H. I. Swanson U. S. Pat. No. 4,580,111 and 4,949,050 disclose an amplitude modulator for use in AM radio broadcasting and wherein the modulator serves to generate an amplitude modulated carrier signal by selectively turning on and off a plurality of RF amplifiers in a digital manner to produce amplitude modulation. Each of the RF amplifiers includes a plurality of switching transistors, each of which may take the form of a MOSFET transistor, connected together in a bridge circuit. This bridge circuit provides output signals to an output combiner. Each of the MOSFET transistors has a gate which is driven by properly phased RF frequency signals that allow the proper MOSFET transistors to be turned on at the correct times. In the arrangement noted in the Swanson patents, the bottom two MOSFET transistors in each bridge circuit are turned on and off to allow each RF amplifier to be turned on or off.
The drive system for driving the RF amplifier MOSFET switching transistors includes a transformer with its own secondary for driving each MOSFET switching transistor. This provides a low impedance source of drive for the gate of each MOSFET switching transistor. This also provides the correct out-of-phase drive to the MOSFET switching transistors. Thus, the bridge arrangement includes upper or high side MOSFET switching transistors and lower or low side MOSFET switching transistors. The correct out-of-phase drive to the MOSFET transistors provides the proper gate voltage with respect to the source voltage. The source voltage may well be on the order of 230 volts DC. For the low side MOSFET transistor drive, the secondary of the transformer is used in conjunction with control transistors to turn the drive on and off to the lower pair of MOSFET switching transistors and which subsequently turn the RF amplifier, itself, on and off. This is discussed in greater detail in the Swanson patents referred to above.
The use of RF transformers for providing the MOSFET transistor drive requires an RF drive signal to the input of each of the MOSFET transistor driver transformers. For proper operation across the frequencies of operation, the input circuits for the RF amplifiers are broad-banded. Consequently, in order to drive the RF amplifier MOSFET switching transistors properly, an RF drive system has been required in previous systems in order to employ a low level oscillator signal at the carrier frequency and then provide enough stages of gain (through tuned circuits and the like) to provide a low impedance drive to the RF amplifier input circuits. In each of the stages, there are losses in the amplifiers, the tuned circuits, and in the coupling circuits employed.